ACPI: battery: make function __battery_hook_unregister() static

The function __battery_hook_unregister is local to the source and does
not need to be in global scope, so make it static.

Cleans up sparse warning:
drivers/acpi/battery.c:654:6: warning: symbol '__battery_hook_unregister'
was not declared. Should it be static?
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/battery.c

@@ -651,7 +651,7 @@ static LIST_HEAD(acpi_battery_list);
 static LIST_HEAD(battery_hook_list);
 static DEFINE_MUTEX(hook_mutex);
 
-void __battery_hook_unregister(struct acpi_battery_hook *hook, int lock)
+static void __battery_hook_unregister(struct acpi_battery_hook *hook, int lock)
 {
 	struct acpi_battery *battery;
 	/*